Don't Skip Soybean Aphid Scouting Despite Low Numbers

The soybean aphid has just now made an appearance in Ohio's soybean fields and growers should begin scouting their fields to monitor population levels.

Ron Hammond, Ohio State University entomologist, said that the insect was found in northeast Ohio counties two weeks ago, but populations so far appear to be lower than what was anticipated.

"We are finding the insect on individual plants rather than throughout a field," said Hammond. "This time last year we were finding thousands of soybean aphids on a single leaflet, so we are not exactly sure why they have not built up in numbers, at least not yet."

The presence of the soybean aphid marks the third year it has been found in Ohio, and each season has brought more questions rather than answers in the challenge to effectively control the insect.

"We haven't had any two summers similar to one another," said Hammond. "Last year populations were so high that the aphids were taking flight in swarms. And this year so far, we are barely finding the insects on a single plant."

Based on research conducted in other states experiencing soybean aphid problems, Hammond and his colleagues speculated that the best time to control the soybean aphid using insecticides was at flowering or immediately thereafter. That is the time when insect populations begin to increase and potentially injure the crop.

The soybean crop, however, is currently flowering with little or no feeding from soybean aphids and Hammond is at a loss to explain the low population levels.

"It's purely a guess, but the low populations could be the result of everything from a mild winter to natural enemies to the wet spring that may have increased naturally occurring pathogens to the hot weather in June," said Hammond. "I think that we are still in a steep learning curve."

The one thing that Hammond does stress is that growers not assume populations will remain low all season.

"The populations could be shifting and we may see high numbers in August. Plus with the late planting, the wet spring and now drought conditions, it's just hard to say what will happen," said Hammond. "The thing that growers must be aware of is that the soybean aphid is here and populations could increase at anytime."

Growers can best check for soybean aphids by scouting 20 or 30 locations throughout their fields and looking for the insect on the uppermost leaflets of the plants.

"Growers don't have to spend a lot of time in their fields. They can get a good feel of soybean aphid levels just by doing a quick walk-through," said Hammond. "Plants with 30 to 100 aphids per leaflet should be treated."

The soybean aphid, the first known aphid to colonize soybeans, is a relatively new insect that has spread throughout the Midwest. States that reported soybean aphid populations last year included Ohio, Michigan, Illinois, Minnesota, North Dakota, South Dakota, Indiana, Kentucky, Iowa and Wisconsin. The insect has also been reported in Virginia, West Virginia, New York and Ontario, Canada.

Researchers know very little about the soybean aphid -- what exactly is its overwintering host, whether or not it's a vector for disease, and why larger insect populations tend to be found in more northern states.

But what they do know is that high insect populations can cause significant yield loss to the crop. Some states reported an average six to eight bushel loss per acre last year due to soybean aphid feeding. In Ohio that yield loss ranged from five to 10 bushels per acre.